It doesn't really matter i suppose, some people have their own way of doing things. It can depend on the size of the job and how much prep is needed.
Obviously if you lay kerbs you need to leave them alone and not be bumping them with a vibrating plate. But on some smaller jobs you may not have time to let them set in which case i can see the sense in doing it after.
Personally i find it easier and more logical setting my kerbs to the desired levels first and then in filling.
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